Mammalian Evolution

New Specimens of the Late Cretaceous Metatherian Eodelphis and the Evolution of Hard-Object Feeding in the Stagodontidae Abstract The Stagodontidae include the largest metatherians known from the Cretaceous of North America. Of the recognized species of the stagodontid genus Eodelphis, E. cutleri is larger and has a more robust dentary, more inflated premolars, and third premolars specialized for crushing, as opposed to the more gracile E. browni. These differences have led to the hypothesis that an E. cutleri-like ancestor gave rise to Didelphodon—another, mostly younger, stagodontid, which has been interpreted as a durophagous predator-scavenger. If correct, E. cutleri would be expected to show more morphological adaptation toward durophagy than E. browni does. Here, we describe two new dentary fossils referable to E. browni and test the evolutionary hypothesis by applying beam theory to estimate bending force capabilities of 22 dentaries of Cretaceous stagodontids and other metatherians. The resulting diversity of bending force profiles of the sampled dentaries implies that Cretaceous metatherians had a wide range of feeding behaviors. Among the stagodontids, E. cutleri has a mediolateral bending force profile of the dentary that is more similar to that of Didelphodon than it is to that of E. browni; whereas its dorsoventral bending force profile is more similar to that of E. browni. These results indicate that anteriorly the dentary of E. cutleri was capable of resisting high torsional stresses from hard-object feeding but lacked other dorsoventral buttressing associated with exceptionally high bite forces of Didelphodon. Our results imply that some morphological changes associated with durophagy evolved twice within this clade, independently in E. cutleri and Didelphodon.

Evolution of the Carnassial in Living Mammalian Carnivores (Carnivora, Didelphimorphia, Dasyuromorphia): Diet, Phylogeny, and Allometry Abstract Different living mammals have developed a carnivorous habit (e.g., Carnivora, Dasyuridae, Thylacinidae, some Didelphidae). They exhibit different specializations for carnivory; however, they share some characters such as a carnassial molar. Previous studies have correlated the shape of molars with diet using morphometric indices or surface scans. In this work, we used 3D geometric morphometrics to explore the shape of the lower carnassials of 235 specimens corresponding to 71 extant species of Carnivora and six extant species of Marsupialia, both Didelphimorphia and Dasyuromorphia. We statistically estimated the effect of size, diet, and phylogeny on molar shape. All the analyses indicated a higher correlation between diet and molar shape, and a better correlation between molar shape and the position of each species on the phylogeny. Therefore, if we take into account the phylogenetic pattern, we can use molar morphology to infer diet of fossil species. Finally, this work evaluates for the first time, in a quantitative way, which of the lower molars of the Metatheria (m3 or m4) is the best analogue to the m1 of Carnivora; our results indicated the m4 is the best analogue.

The Morphology of the Bovid Calcaneus: Function, Phylogenetic Signal, and Allometric Scaling Abstract Despite its clear functional role in hock (ankle) plantarflexion, the bovid calcaneus has been understudied with respect to the functional constraints imposed by locomotion in differing habitats, the allometric influence of inter-specific body size differences, and phylogenetic signal. This study uses a comparative sample of extant bovid species to shed light on the evolution of bovid calcaneal morphology. I measured eight linear measurements on 204 calcaneus specimens representing 41 extant bovid species. Using a morphological body size proxy validated against published species-mean body mass estimates, I performed Ordinary Least Squares regression to examine the allometric relationships of each measurement with body size. I classified each bovid species to a preferred habitat type based on published literature, and performed Phylogenetic Generalized Least Squares (PGLS) to test for differences in morphology between bovid taxa with different preferred habitats while considering evolutionary relatedness. I visualized morphological differences between taxa using Principal Components Analysis plotted in a phylomorphospace. Results demonstrate that several measurements of the bovid calcaneus have an allometric relationship to body size. The functional length of the calcaneus scales with negative allometry, which likely maintains a comparable safety factor within the calcaneal tuber at larger body sizes. While open-habitat bovids have relatively shorter calcaneal tubers, this difference is not significant when controlling for the influence of body size and phylogenetic signal using PGLS. Among bovid tribes that have a deep evolutionary history of adaptation to open habitats, Antilopini have relatively longer calcaneal tubers than Alcelaphini or Hippotragini, which may reflect the unique importance of stotting behavior in predator avoidance among antelopins. Overall, the morphology of the bovid calcaneus has been shaped by a complex interaction of phylogenetic and body-size constraints as well as adaptation to modes of predator avoidance mediated by preferred habitat.

New Early Miocene Material of Iberictis , the Oldest Member of the Wolverine Lineage (Carnivora, Mustelidae, Guloninae) Abstract We describe new dental remains of the genus Iberictis (Carnivora: Mustelidae) from the late early Miocene of the Iberian Peninsula. The new fossils of Iberictis azanzae from Artesilla (16.5–16.3 Ma, MN4; Calatayud-Teruel Basin, Zaragoza, Spain) add important morphological information about this species. Material from another species, Iberictis buloti, is described from els Casots (16.5–16.3 Ma, MN4; Vallès-Penedès Basin, Barcelona, Spain). This material constitutes the most complete sample of Iberictis and the first record of I. buloti in the Iberian Peninsula. Our analyses confirm the taxonomic validity of both species, and confirm the more plesiomorphic status of I. buloti compared to I. azanzae. Re-examination of large mustelid Miocene genera (Dehmictis, Ekorus, Eomellivora, Hoplictis, Iberictis, Ischyrictis, and Plesiogulo) and their inclusion for the first time in a cladistic analysis indicate that Iberictis is the sister taxon of Plesiogulo, and that these genera constitute the sister group of the extant wolverine (Gulo gulo). Our analysis thus confirms a close relationship between the early Miocene Iberictis, the late Miocene Plesiogulo, and the Plio-Pleistocene Gulo. Iberictis is the oldest member of Gulonini, the total clade of wolverines, thereby tracking the fossil record of this clade back to the early Miocene. We further propose a new systematic arrangement for the aforementioned large Miocene mustelids into the subfamilies Guloninae, Mellivorinae, and Mustelinae.

New Metatherian Mammal from the Early Eocene of Antarctica Abstract A new Paleogene metatherian from locality IAA 1/90, Marambio (Seymour) Island in the Antarctic Peninsula is described. Pujatodon ektopos, gen. et sp. nov., is recognized on the basis of a tiny lower left molar recovered from early Eocene (late Ypresian) levels of the Cucullaea I Allomember, La Meseta Formation. The tooth is characterized by its small size, bunoid aspect, short trigonid with closely set paraconid and metaconid, wide and long talonid, the development of an incipient cingulid at the labial base of the crown between the trigonid and talonid, and an expanded posterior cingulid. Body mass estimations for the new taxon range from 83.13 to 153.15 g. Its enamel microstructure shows the earliest evidence among metatherians of aligned prisms, as well as of interprismatic sheets of matrix. The analysis of other parameters, like body mass, molar morphometric index, and wear facets, suggests that the molars of Pujatodon were primarily adapted to the processing of fruits, nuts, seeds, and/or hard insects. Several features suggest the allocation of this specimen among basal polydolopimorphians (Prepidolopidae or, more probably, Glasbiidae). The discovery of the specimen MLP 14-I-10-20 could offer new insights on the origins and early diversification of Australidelphian marsupials in southern (and northern?) continents. It also adds significant information on the diversity of Antarctic Paleogene mammals, their evolution, habits, and historical biogeography.

Lineages of Tuco-Tucos (Ctenomyidae: Rodentia) from Midwest and Northern Brazil: Late Irradiations of Subterranean Rodents Towards the Amazon Forest Abstract The genus Ctenomys comprises approximately 70 recognized living species of subterranean rodents endemic to South America. Phylogenetic studies to date, based on mitochondrial DNA data, place 44 recognized species in eight species groups and provide evidence for a burst of speciation early in the history of the genus. Species from Argentina, Uruguay, Bolivia, and southern Brazil are well studied at the phylogenetic level. However, the taxonomic status of the species inhabiting midwest and northern Brazil remains poorly understood. In this study, we construct phylogenies based on maximum likelihood and Bayesian inference methods with cytochrome b gene haplotypes of Ctenomys from midwest and northern Brazil and with haplotypes representative of the genus Ctenomys to place the sampled haplotypes into a phylogenetic framework; we also evaluate skull geometric morphometrics data among sampling sites to assess whether skull morphology corroborates the phylogenetic patterns observed. The results show that the sampling sites used in this study are represented by two species, namely, Ctenomys bicolor, which is present in the state of Rondônia, and Ctenomys nattereri, which is present in Mato Grosso and Bolivia. The results also reveal two lineages of Ctenomys distinct from C. bicolor and C. nattereri, henceforth called Ctenomys sp. “xingu” and Ctenomys sp. “central.” Both the species and lineages share a most recent common ancestor with C. boliviensis and are part of the boliviensis species group.

Australogale leptognathus , gen. et sp. nov., a Second Species of Small Sparassodont (Mammalia: Metatheria) from the Middle Miocene Locality of Quebrada Honda, Bolivia Abstract The Sparassodonta (Mammalia: Metatheria) were the principal group of carnivorous mammals in Cenozoic South America and an important component of this continent’s terrestrial predator guild for nearly 60 million years. However, knowledge of the evolutionary history of this group is biased towards species larger than 1.5 kg from extra-tropical latitudes. Here, we describe a new, small sparassodont from the late middle Miocene (Serravallian) of Quebrada Honda, Bolivia, a locality in the middle latitudes (southern tropics) of South America (~ 22° S). This species, Australogale leptognathus, gen. et sp. nov., is represented by a partial, well-preserved dentary and can be distinguished from other sparassodonts by a combination of features including a p3 that is much smaller than p2, absence of a posterobasal heel on p2, a conical entoconid, absence of an entocristid on m1, and an entocristid on m2 that is lingual to the trigonid. This new species is among the smallest Neogene sparassodonts, with an estimated body mass of ~840 g. Australogale leptognathus expands the known taxonomic and morphological diversity of late middle Miocene sparassodonts and provides evidence that the group may have been as diverse during the late middle Miocene as during the late Oligocene and early Miocene.

Digital Cranial Endocasts of the Extinct Sloth Glossotherium robustum (Xenarthra, Mylodontidae) from the Late Pleistocene of Argentina: Description and Comparison with the Extant Sloths Abstract The internal cranial morphology of the terrestrial sloth Glossotherium robustum is described here based on a neurocranium from the late Pleistocene of the Pampean region of Buenos Aires, northeastern Argentina. The first published data on the morphology of the brain cavity of this species date back to the latest nineteenth century. The novel techniques of CT scanning and digital reconstructions enable non-destructive access to the internal cranial features of both extinct and extant vertebrates, and thus improve our knowledge of anatomical features that had previously remained obscure. Therefore, we performed CT scans on the posterior half of a skull of G. robustum and created digital models of the endocasts and internal structures. The results reveal the morphology of the brain cavity itself, as well as the paranasal sinuses and the trajectory of several cranial nerves and blood vessels. These features have been compared with the two extant folivoran genera, the two-toed sloth Choloepus and the three-toed sloth Bradypus. For many characteristics, especially those related to the paranasal pneumaticity and the brain cavity, a closer similarity between Glossotherium and Choloepus is observed, in accordance with the most widely accepted phylogenetic scenarios. However, other features are only shared by the two extant genera, but are probably related to allometric effects and the convergence that affected the two modern lineages. This study, which represents the first exhaustive analysis of digital endocasts of a fossil sloth, reveals the importance of the application of new methodologies, such as CT scans, for elucidating the evolutionary history of this peculiar mammalian clade.

Evolution of Body Mass in Bats: Insights from a Large Supermatrix Phylogeny Abstract Bats are atypical small mammals. Size is crucial for bats because it affects most aerodynamic variables and several key echolocation parameters. In turn, scaling relationships of both flight and echolocation have been suggested to constrain bat body size evolution. Previous studies have found a large phylogenetic effect and the inclusion of early Eocene fossil bats contributed to recover idiosyncratic body size change patterns in bats. Here, we test these previous hypotheses of bat body size evolution using a large, comprehensive supermatrix phylogeny (+800 taxa) to optimize body size and examine changes reconstructed along branches. Our analysis provides evidence of rapid stem phyletic nanism, an ancestral value stabilized at 12 g for crown-clade Chiroptera followed by backbone stasis, low-magnitude changes inside established families, and massive body size increase at accelerated rate in pteropodid subclades. Total variation amount explained by pteropodid subclades was 86.3%, with most changes reconstructed as phyletic increases but also apomorphic decreases. We evaluate these macroevolutionary patterns in light of the constraints hypothesis, and in terms of both neutral and adaptive evolutionary models. The reconstructed macroevolution of bat body size led us to propose that echolocation and flight work as successive, nested constraints limiting bat evolution along the body size scale.

Unraveling Deep Branches of the Sigmodontinae Tree (Rodentia: Cricetidae) in Eastern South America Abstract Neotropical cricetid rodents of the subfamily Sigmodontinae constitute an incredibly diverse and ubiquitous clade of mammals in South America. In spite of recent advances in sigmodontine systematics, a variable collection of genera from eastern South America with no clear tribal affiliations and well-differentiated morphologies has been pervasive in most classification attempts, being generally listed as Sigmodontinae incertae sedis. Here, we assess the phylogenetic relationships of these incertae sedis genera based on supermatrix and multispecies coalescent analyses of a multilocus molecular dataset (one mitochondrial and five nuclear loci) for 76 genera, calibrated with ten fossil-based node dates. Both analytical approaches provided concordant and unprecedented resolution of suprageneric clades within the large Oryzomyalia clade (constituted by most sigmodontine genera), recovering a natural group constituted by the genera Wiedomys, Wilfredomys, and the Atlantic Forest endemics Juliomys and Phaenomys - here redefined as the Wiedomyini tribe - and a sister-relationship between this group and the tribe Abrotrichini. The enigmatic spiny rodent genus Abrawayaomys was recovered as sister to Akodontini in three of four analyses, but pending higher support. Morphological comparisons performed among the four wiedomyine genera highlighted a number of shared external, cranial, and dental similarities useful to emend the tribe diagnosis, such as the presence of chromo-genetic fields associated with the muzzle, pinnae, and rump, the long and narrow rostrum flanked by shallow and narrow zygomatic notches, and the asymmetrically divided procingulum of M1, among other characters. Molecular dating coupled with fossil evidence suggests Wiedomyini as one of the oldest sigmodontine tribes with a late Miocene autochthonous origin in eastern South America, highlighting the relevance of this region as an ancient cradle of sigmodontine lineages.